1. Field of the Invention
The present invention relates to speed reducers and, more specifically, to a compound epicyclic speed reducer having a high velocity ratio, such as at least 1,000:1, and usable for an extended period at high input speed for the driving shaft, such as at least 3,000 revolutions per minute.
2. Prior Art
A speed reducer having a high velocity ratio between the driving and driven components of the reducer is required in applications where it is desired to increase the output torque of the driven component greatly over the input torque of the driving component, or where it is desired to decrease the output speed of the driven component greatly over the input speed of the driving component.
For achieving a high velocity ratio, "ordinary" gear trains, that is, trains using gears rotatable about axes stationary relative to each other, are undesirable because many sets of gears in series are required to achieve a high velocity ratio. While a higher velocity ratio can be achieved with fewer parts by use of worms, worms require precise machining and, in general, have a lower power transmission efficiency than spur gears.
Epicyclic gear trains offer the advantage of requiring fewer sets of gears to achieve a high velocity ratio, resulting in less weight without substantial reduction in power transmission efficiency as compared to ordinary gear trains. However, the gears of epicyclic speed reducers must be precisely machined and precisely positioned. In addition, there is more difficulty in adjusting the velocity ratio from any designed value for an epicyclic speed reducer than for an ordinary speed reducer because changing the size of any one gear in an epicyclic speed reducer usually requires repositioning at least some of the other gears.
Some of the problems with epicyclic speed reducers can be overcome by exotic design. For example, in the device of Baron U.S. Pat. No. 3,481,222 a "floating" orbiting planet shaft is provided so that the planet gears need not be positioned quite so precisely with respect to the sun gears as in an epicyclic speed reducer having a firmly positioned planet shaft.
A problem common to both ordinary and epicyclic speed reducers using meshing gears is that maximum input speed is limited. In producing electric motors of a given power rating, for example, the power to cost and power to weight ratios are substantially higher for a high-speed motor, such as a motor having an output speed in the range of 3,000 to 20,000 rpm, than for a low-speed motor, such as a motor having an output speed in the range of 1,000 to 3,000 rpm, so that an inexpensive high velocity ratio speed reducer capable of handling high input speed would be desirable. Conventional speed reducers using meshing gears simply cannot be driven above about 3,000 rpm for extended periods because of heat build-up resulting in rapid wear--at least without the additional expense of high precision machining and/or exotic and expensive gear materials and lubricants or lubrication systems. At high speed, lubricant is flung from the gears by centrifugal force. Heat buildup and rapid wear at high speed is even more of a problem with worms. A practical upper limit for the input speed of worm gear reducers is about 2,000 rpm.
Lubrication problems also can be overcome somewhat by exotic design. For example, the "Hart reduction pulley", which has a meshing gear epicyclic reducer, uses "splash lubrication" in which lubricant flung from spinning gears contacts the inner periphery of the specially designed reducer housing and splashes back onto the rotating gears. Nevertheless, even the Hart reduction pulley is recommended for use with a motor having an output speed of only 1,750 rpm.
Naturally, total cost is a primary consideration in the selection of an electric motor-speed reducer combination producing a desired output speed and torque. In general, where low output speed is required, the cost of the speed reducer portion is much greater than the cost of the electric motor portion. Up to now, to achieve a desired low output speed, the combination of a high-speed motor and a high velocity ratio speed reducer capable of handling high input speed has not been cost effective, because the additional expense of the high input speed, high velocity ratio speed reducer is substantially greater than the cost savings resulting from using a high-speed motor. Again, it is apparent that an inexpensive high velocity ratio speed reducer capable of handling high input speed is desirable.
For a specific application, ordinary and epicyclic gear trains can be used in combination. For example, the device of Zucchellini Canadian Pat. No. 824,402 uses an epicyclic reducer driving a worm to achieve exact positioning of a slide that can carry machine tools. The devices of Lemmens Canadian Pat. No. 922,926 and Roper Canadian Pat. No. 935,668 use variable pitch pulley belt drives connected to the input shaft or the input and output shafts of an epicyclic speed reducer for providing an infinitely variable transmission.
Ordinary and epicyclic gearing also can be connected in series to overcome a disadvantage of either system when used alone. In the Hart reduction pulley, an initial speed reduction is achieved by an open V-belt drive connecting the output pulley of a motor and the input pulley of an epicyclic speed reducer. The result is that the total velocity ratio can be altered through a limited range by changing the velocity ratio of the open V-belt drive without changing the velocity ratio of the epicyclic speed reducer.
The Hart reduction pulley falls into a further class of speed reducers that includes the devices of Helling Canadian Pat. No. 456,655 and the following United States patents:
Morini U.S. Pat. No. 3,115,794; PA1 Philpott et al. U.S. Pat. No. 3,842,685; and PA1 Lee U.S. Pat. No. 4,044,633.
Each of these devices uses some type of endless loop force-transmitting element, such as a belt or a chain, in conjunction with an epicyclic speed reducer. Also, in each of these devices input power is applied directly to the carrier member for the orbiting planet shaft of the epicyclic reducer. An advantage of using endless loop force-transmitting elements that is not recognized in any of these patents is that higher input speed can be accommodated by speed reducers using endless loop force-transmitting elements rather than meshing gears; and a disadvantage of each of these devices that is not recognized in any of the patents is the decreased maximum input speed permitted by driving a carrier member directly.